1. Field of the Invention
The present invention relates to a temperature control system for a grating used in dispersion equalizing in an ultra-high-speed optical communication system. Particularly, the temperature control system includes a temperature control device for a grating, a method of storing a temperature control pattern in a storage device, a method of automatically controlling the temperature control device for a grating, and a variable dispersion equalizer.
2. Description of the Prior Art
In an optical communication system using an optical fiber cable as a transmission path, since an optical pulse is distorted by wavelength dispersion (also called dispersion, to be referred to as xe2x80x9cdispersionxe2x80x9d hereinafter) of the optical fiber, a signal is degraded. A reason why dispersion occurs when the optical fiber cable is used as described above will be described below. In a material constituting a general optical fiber cable, a group velocity of a wave packet of optical pulses depends on a wavelength, and a time required to propagate the wave packet, i.e., a group delay time (unit: ps) changes. An inclination of the group delay time to the wavelength is dispersion (unit: ps/nm). In a single mode fiber used in a general optical fiber transmission path, dispersion generated for a transmission path of 1 km has a value of about 16 ps/(nmxc2x7km) at a wavelength of about 1550 nm. This means that the difference between delay times required to propagate optical pulses having wavelengths which are different from each other by 1 nm through a single mode fiber (hereinafter referred to as SMF) of 1 km is 16 ps. For example, a group delay time when optical pulses having wavelengths which are different from each other by 1 nm are propagated through an optical fiber cable of 100 km is 1600 ps which is 100 times the group delay time obtained in the above case.
On the other hand, modulated optical pulses have the spreads of several spectra determined by a modulation method or a bit rate, and an envelope for the optical pulses is of a Gaussian distribution type. For example, in an RZ (return-to-zero) modulation method, when a bit rate (transmission speed) is 10 Gbit/s, intervals between the respective line spectra are 0.08 nm each. However, when the bit rate is 40 Gbit/s, intervals are 0.32 nm each. More specifically, the spread of the line spectrum increases in proportion to a bit rate. In an NRZ (non-return-to-zero) modulation method, the spread of a line spectrum is half the spread of the line spectrum in the RZ modulation method. In this manner, as a bit rate increases, the interval between line spectra which are the components of optical pulses increases. For this reason, the difference between group delay times when the optical pulses are propagated through an optical fiber transmission path increases, distortion of the optical pulses increases. In addition, an influence of a dispersion of an optical fiber transmission path received by optical pulses increases in proportion to the square of a bit rate. For this reason, a device having dispersion which cancels the dispersion of the optical fiber transmission path is inserted into the transmission path, and the dispersions are made close to zero. This technique is a dispersion compensation technique. In particular, a dispersion of a transmission path at a bit rate of 40 Gbit/s or more must be made precisely close to zero. At a bit rate of 80 Gbit/s or more, a dispersion slope which is a rate of a change in dispersion caused by a wavelength must be compensated for.
As a device for equalizing such a dispersion, a variable dispersion equalizer using a chirp grating is known. For example, as shown in a perspective view in FIG. 19, Japanese Laid-Open Patent Publication No. 10-221658 discloses a variable dispersion equalizer using a chirp grating. In a fiber grating 1 serving a chirp grating, circularly cylindrical compact thick-film heaters 31, 32, . . . , 3N (N is an integer) consisting of tungsten, NiCr, or the like are arranged in a capillary 2 such as a hollow ceramics consisting of an insulator and having a through hole for fixing a fiber having a relatively large diameter such that the circularly cylindrical compact thick-film heaters 31, 32, . . . , 3N are coaxial with the through hole of the capillary 2. Here, the heaters 31, 32, . . . , 3N are arranged at equal intervals in the longitudinal direction of the capillary 2. When currents are flowed into the heaters 31, 32, . . . , 3N such that the currents increase by a predetermined value, the fiber grating 1 is gradually heated in a micro-section, but is heated with a predetermined temperature gradient as a whole. The equivalent refractive index of the fiber grating 1 changes depending on an applied voltage to realize a linear chirp characteristic. The equivalent refractive index is also called an effective refractive index, is an equivalent refractive index which is received by light propagated through an optical fiber cable, and is a refractive index generated by an interactive function between the refractive indexes of a core and a cladding and a propagation path of light. Although, exactly, the grating pitch of the fiber grating 1 also changes depending on a change in temperature, the change of the grating pitch is neglected because the influence of the change of the grating pitch is smaller than that of the change of the equivalent refractive index.
There was no temperature control device for grating which appropriately controlled a plurality of heaters disposed near the grating to give an appropriate temperature distribution to the grating. More specifically, when powers applied to the heaters disposed near the grating are not appropriately controlled, a temperature distribution given to the grating is incorrect to adversely affect chirp characteristics such as a dispersion and a dispersion slope given to a reflected light component. In this case, a group delay ripple which is a shift from an almost linear relationship between a group delay time and a wavelength is generated. On the other hand, when a temperature distribution given from each heater to the grating is a linear distribution, a group delay ripple caused by a manufacturing error inherent in the grating may occur. In addition, in a conventional variable dispersion equalizer, a group delay ripple which adversely affects transmission quality occurs due to a gradual temperature distribution generated by the plurality of heaters disposed near the grating. In addition, the cycle of the group delay ripple is dependent on the numeral distribution of the heaters. The existence of the group delay ripple having a predetermined cycle or more considerably influences the numeral distribution of the heaters at a high bit rate.
In a variable dispersion equalizer disclosed in Japanese Laid-Open Patent Publication No. 10-221658, a case in which a linear chirp characteristic is given exemplified. However, control of each heater is not concretely described. The heaters are arranged at equal intervals in the longitudinal direction, and the numeral distribution of the heaters is not considered. The cycle of a group delay ripple is not described.
Therefore, the first object of the present invention to provide a system of variable dispersion equalizer with grating, which gives a predetermined dispersion and a dispersion slope with suppressed a group delay ripple inherent. It is the second object of the present invention to provide a variable dispersion equalizer having the small cycle of a group delay ripple.
In accordance with one aspect of the present invention, there is a temperature control device for controlling a plurality of temperature variable device for grating which are disposed near the grating of a variable dispersion equalizer constituted by an optical waveguide forming the grating and the temperatures of which can be independently controlled. The temperature control device includes a controller for controlling the plurality of temperature variable device. In addition, the controller controls the temperature variable device by a control signal of at least one of the temperature control patterns constituted by combinations of control signals of the temperature variable device.
In another aspect of the present invention the temperature control device further includes a storage device in which a plurality of temperature control patterns constituted by combinations of control signals of the temperature variable device are stored. In addition, the controller controls the temperature variable device by the control signal of at least one of the temperature control patterns selected from the storage device.
In a further aspect of the present invention, the controller includes at least two control signal setting devices and a signal adding device. The at least two control signal setting device set the control signals applied to the plurality of temperature variable device. The signal adding device adds the signals from the respective control signal setting device to transmit added signals to the respective temperature variable device. In addition, the temperature variable device are controlled by the added control signals.
The two control signal setting devices may set a dispersion and a dispersion slope of the grating.
Also, the control signal setting device may further include a control signal setting device for canceling a group delay ripple of the grating.
In a yet further aspect of the present invention, the control signal setting device further includes control signal setting device for applying a constant bias temperature to the entire grating.
The control signal setting device may be constituted by a variable resistor group.
In a yet further aspect of the present invention, the temperature control device further includes a storage device in which a plurality of temperature control patterns constituted by combinations of the control signals of the temperature variable device are stored. In addition, the control signal setting device is set by the control signal of at least one of the temperature control patterns selected from the storage device.
In a yet further aspect of the present invention, a storing method is performed to store the plurality of temperature control patterns constituted by combinations of control signals for controlling the plurality of temperature variable device in the storage device. The storing method includes the following steps of:
STEP 1: applying predetermined initial control signals to the temperature variable device such that a group delay time characteristic of the grating is a predetermined characteristic;
STEP 2: measuring the group delay time characteristic of the grating to compare the measured group delay time characteristic with the predetermined group delay time characteristic;
STEP 3: correcting the initial control signal such that a group delay ripple which is a difference between the measured group delay time characteristic and the predetermined group delay time characteristic is not more then an allowable value to calculate a corrected control signal;
STEP 4: applying the corrected control signal to the respective temperature variable device; and
STEP 5: storing a combination of the corrected control signal applied to the respective temperature variable device and the measured group delay time characteristic in storage device.
In addition, the steps are repeated to store the plurality of temperature control patterns in the storage device.
In a yet further aspect of the present invention, a storing method is performed to store the plurality of temperature control patterns constituted by combinations of control signals for controlling the plurality of temperature variable device in the storage device. The storing method includes the following steps of:
STEP 6: causing an operation device to calculate an initial control signal applied to the temperature variable device such that a group delay time characteristic of the grating has a predetermined value;
STEP 7: applying the initial control signal to the respective temperature variable device;
STEP 8: measuring the group delay time characteristic of the grating to compare the measured group delay time characteristic with the predetermined group delay time characteristic;
STEP 9: obtaining the predetermined group delay time characteristic when a group delay ripple which is a difference between the measured group delay time characteristic and the predetermined group delay time characteristic is large, and performing at least once following the sub-steps of:
SUB-STEP 1: causing an operation device to calculate a corrected control signal applied to the temperature variable device;
SUB-STEP 2: applying the corrected control signal to the respective temperature variable device; and
SUB-STEP 3: measuring the group delay time characteristic of the grating to compare the measured group delay time characteristic with the predetermined group delay time characteristic; and
STEP 10: storing combination of the corrected control signal applied to the respective temperature variable device and the measured group delay time characteristic in storage device when a ripple which is a difference between the measured group delay time characteristic and the predetermined group delay time characteristic is within an allowable range.
In addition, the above steps are repeated to store the plurality of temperature control patterns in the storage device.
In a yet further aspect of the present invention, the controller includes a photoelectric conversion unit and an operation device. The photoelectric conversion unit photoelectrically converts an optical signal reflected by the grating of the optical waveguide. The operation device compares the photoelectrically converted electric signal with a predetermined value to apply the electric signal to the respective temperature variable device such that the electric signal has a value which is not less than the predetermined value.
In a yet further aspect of the present invention, the photoelectrically converted signal is a clock voltage.
In accordance with one aspect of the present invention, there is a method of automatically controlling a temperature control device for controlling a plurality of temperature variable device for grating which are disposed near a grating of a variable dispersion equalizer constituted by an optical waveguide forming the grating and the temperatures of which can be independently controlled. The method includes the following steps of:
STEP 11: converting an optical signal reflected by the grating into an electric signal;
STEP 12: comparing the electric signal with a predetermined value;
STEP 13: adjusting a control signal applied to the respective temperature variable device such that the electric signal has a value which is not less than the predetermined value; and
STEP 14: controlling the respective temperature variable device by the control signal.
In another aspect of the present invention, the temperature variable device is heater, and the electric signal is a clock voltage. In addition, the step of converting includes the following steps of:
STEP 15: acquiring the clock voltage for each predetermined time;
STEP 16: comparing the clock voltage with a predetermined voltage;
STEP 17: changing voltages of a plurality of heaters such that the clock voltage is maximum when the clock voltage has a value which is not more than the predetermined voltage, and performing at least once following the sub-steps of:
SUB-STEP 4: changing the voltages of the respective heaters by a predetermined value; and
SUB-STEP 5: comparing clock voltages with each other before and after the sub-step.
In accordance with one aspect of the present invention, there is a variable dispersion equalizer includes an optical waveguide and a plurality of temperature variable device. The plurality of temperature variable device are disposed near the grating and can be independently controlled with respect to the temperature. In addition, the plurality of temperature variable device are arranged at a numeral distribution defined by a grating pitch xcex9(l) which is a function of a length l of the grating in the longitudinal direction and an equivalent refractive index Neff(l).
In another aspect of the present invention, the plurality of temperature variable device are arranged such that a numeral distribution of the temperature variable device satisfies a relationship of (the number of temperature variable device per unit length, n) as follows:
nxe2x89xa72xcex94/0.1 
Note that n is the number of temperature variable device (the number of temperature variable device/unit length) in a longitudinal direction of the optical waveguide. It is noted that a difference between products xcex9xc2x7Neff of the grating pitches xcex9 and the equivalent refractive indexes Neff in unit lengths in the longitudinal direction of the optical waveguide is represented by xcex94.
In a further aspect of the present invention, the grating is a chirp grating the grating pitch of which is changed in the longitudinal direction of the optical waveguide.
According to a temperature control device of the present invention includes a storage device which stores a plurality of temperature control patterns includes combinations of control signals for controlling a plurality of temperature variable device the temperatures of which can be independently controlled are stored. By the temperature control patterns, the plurality of temperature variable device disposed near a grating of an optical waveguide constituting a variable dispersion equalizer are controlled, so that a predetermined temperature distribution can be given to the grating. A predetermined dispersion and a predetermined dispersion slope can be given to the grating.
According to a temperature control device of the present invention includes at least two control signal setting device for setting control signals applied to a plurality of temperature variable device. In this manner, control signals of at least two types are combined to each other, so that the respective temperature variable device can be controlled.
According to the temperature control device of the present invention, two control signal setting device can set a dispersion and a dispersion slope of the grating, respectively.
In addition, according to the temperature control device of the present invention, a group delay ripple of the grating can be canceled by another control signal setting device.
Furthermore, according to the temperature control device of the present invention, a constant bias temperature can be applied to the entire grating by still another control signal setting device.
According to the temperature control device of the present invention, since the control signal setting device is constituted by a variable resistor group, the control signals can be precisely set with a simple configuration.
Since the temperature control device of the present invention further includes a storage device which stores temperature control patterns, calculation or the like of control signals given to the temperature variable device need not be performed, and setting control signals can be easily performed.
According to a method of storing a temperature control pattern in the storage device of the present invention, a temperature control pattern in which a group delay ripple inherent in a grating is corrected in advance is generated, and can be stored in the storage device. Since various temperature control patterns are stored in the storage device in advance, the temperature control patterns can be easily given to the grating when the grating is used.
According to the method of storing a temperature control pattern in the storage device of the present invention, a temperature control pattern in which a group delay ripple inherent in a grating is corrected in advance is generated, and can be stored in the storage device. Since various temperature control patterns are stored in the storage device in advance, the temperature control patterns can be easily given to the grating when the grating is used.
According to the temperature control device of the present invention, a reflected light component reflected by a grating is converted into an electric signal, and control signals applied to the respective temperature variable device are adjusted such that the electric signal has a predetermined value or more.
According to the temperature control device of the present invention, a reflected light component reflected by a grating is photoelectrically converted into a clock voltage. For this reason, dispersion equalization of the grating can be easily observed by a change in voltage.
According to a method of automatically controlling a temperature control device of the present invention, a reflected light component reflected by a grating is photoelectrically converted into an electric signal, and control signals applied to the respective temperature variable device are adjusted, so that the electric signal has a predetermined value or more. In this manner, dispersion equalization of the grating can be optimized by automatic control.
According to the method of automatically controlling a temperature control device of the present invention, a reflected light component reflected by a grating is photoelectrically converted into a clock voltage, and the respective heaters are adjusted such that the clock voltage is maximum in adjustment of each of the heaters. In this manner, since the clock voltage can be adjusted to have a predetermined value or more, dispersion equalization can be optimized by automatic control.
According to a variable dispersion equalizer, a plurality of temperature variable device are arranged at a numeral distribution defined by a grating pitch xcex9(l) which is a function of a length l of the grating in the longitudinal direction and an equivalent refractive index Neff(l). In this manner, an influence of a step-formed temperature distribution generated by heaters is suppressed, a group delay ripple generated in a reflected light component by the grating can be reduced, and the cycle of the group delay ripple can be reduced.
According to the variable dispersion equalizer of the present invention, the plurality of temperature variable device are arranged such that a numeral distribution of the temperature variable device satisfies a relationship of (the number of temperature variable device per unit length, n) as follows:
nxe2x89xa72xcex94/0.1 
In this manner, an influence of a step-formed temperature distribution generated by heaters is suppressed, a group delay ripple generated in a reflected light component by the grating can be reduced, and the cycle of the group delay ripple can be reduced.
In addition, according to the variable distribution equalizer of the present invention, since a chirp grating is used, a predetermined distribution can be given to the grating without giving a linearly functional temperature distribution by the plurality of temperature variable device.